Cathode ray system



March 5, 1963 e. w. PRESTON CATHODE RAY SYSTEM Filed June 22, 1959 r N 7w a an M H 04 s m FF F 3 E a M a w CONTROL VOZI'HGE SOURCE INVENTOR. 6454/4/ M FRESIOA/ iinited States 3,9895% Patented Mar. 5, 1963 3,089,500CATl-IGD RAY SYSTEM Glenn W. Preston, Oreland, Pa, assignor, by mesneassignments, to Philco Corporation, Philadelphia, Pa, a corporation orDelaware Filed lane 22, 195% Ser- No. 822,954 4 Claims. (Cl. 315-46)This invention relates to electron discharge devices and in particularto a novel electron gun for use therein.

The invention is particularly adapted for use with cathode ray tubes andwill be described in connection therewith, although it should beunderstood that numerous other applications thereof are possible. Inconventional cathode ray tubes there is customarily a cathode or otherelectron-emitting structure, a beam accelerating electrode to which ahigh positive potential is applied, a control electrode intermediate thecathode and the accelerating electrode, and an electron-collectingelectrode. Modulation of the beam is usually eifected by supplying anegative voltage wave, which may have amp-ntude values in the range 6 to50 volts for example, to the control electrode. Control voltages of thisorder of magnitude and polarity are necessary to enable the beam to becut oil so as to overcome the influence of the highly positiveaccelerating electrode. The magnitude of this negative cut-off potentialis one important factor in limiting attainable values oftransconductance.

Conventional electron gun structures are also characterized by unevenutilization of the emitting surfac This is caused by the fact that, asthe control signal on the control electrode becomes more positive withrespect to the cathode, electrons from a larger area of the emittingsurface are employed in increasing the beam current. Conversely, whenthe control electrode potential becomes less positive with respect tothe cathode, electrons from a smaller area of the emitting surface,i.e., from more centrally located areas, are used in forming the beam.This phenomenon results from the fact that the contours of the tips ofthe equipotential lines from the positive accelerating anode, whichextend through the control electrode aperture toward the cathode, becomeflatter and are parallel to the emitting area at more points when thepotential on the control electrode becomes more positive, and becomenarrower and more pointed toward the center of the emitting surface whenthe control electrode goes more negative.

It is also a characteristic of conventional electron gun structures thatthe electric held between the cathode and the control electrode forms aconvergence lens which is subject to the electron-optical analogue ofthe optical aberration known as spherical aberration. The lens act-ioncauses electrons from the more peripheral portion of the emittingsurface to crossover at points on the electron-optical axis which aredifferent from the points at which electrons from the central portion ofthe emitting surface crossover. In conventional guns sphericalaberration may also occur elsewhere in the electron-optical system andif, as is usually the case, the spherical aberration is positive (i.e.the peripheral electrons from he cathode cross the electron-optical axisahead of the paraxial electrons), it is difficult to correct orcompensate for it by introducing appropriate amounts of negativespherical aberration.

It is therefore an object of the invention to provide an electron gun oruse in an electron discharge device which permits attainment of highervalues of transc-onductance than are possible with conventional electronguns.

It is a further object of the invention to provide an electron gun for acathode ray tube in which a novel system of beam-modulation is employed.

Another aim of the invention is to provide a cathode ray tube electrongun whose electron-emitting surface is more uniformly employed in theproduction of an electron beam.

Another object of the invention is tron gun for a cathode rayaberrations are reduced or Still another object of the invention is toprovide an electron gun for a cathode ray tube which can be designed tocorrect for positive spherical aberration.

In accordance with the invention the foregoing objects are achieved bymeans of an electron gun in which the intensity of the beam emitted fromthe cathode emitting surface is maintained substantially constant and inwhich modulation thereof is produced by absorbing desired numbers of theelectrons in the beam and reflecting others to an appropriateelectron-collecting body. In one form of the invention I provide acathode for emitting electrons, an electrode assembly for impelling theemitted electrons along a given path, and an electron mirror electrodeto which a relatively low control voltage wave is applied. Thiselectrode absorbs some of the emitted ele trons and reflects others, asa function of the amplitude and polarity of the signal applied thereto,along another path toward a collecting electrode. Thus, as the controlvoltage goes more negative, more of the emitted electrons will bereflected and fewer will be absorbed than when it goes more positive,and vice versa. The electrons which are reflected constitute an electronbeam which is effectively modulated in intensity and which may bedirected onto an appropriate electron-collecting body for utilization.According to one feature of the invention the electrons impelled towardthe electrode are made to trace essentially parallel rays so that theelectron mirror will have substantially the same eifect on all of themand those that are reflected will be reflected in substantially the sameway. To obtain this kind of beam a plurality of positive electrodes aredisposed in proximity to the cathode which cause the electrons from theemissive surface first to diverge and then to converge slightly, therebyproducing substantially parallel electron rays.

The sole FIGURE illustrates a form of the invention as used in a cathoderay tube.

Within an evacuated envelope 10 a cathode 12 of the thermionic-emissivetype is located which is heated by the filament 14 when current isapplied thereto. Electrons emitted from the cathode 12 are attracted byan electrode 16 to which a constant positive potential of about +5 voltsis applied. The electrode 16 may be in the form of a hollow cylinderhaving a closed end containing an aperture 18 through which the emittedelectrons pass. They are then attracted by the electrode 20 which is ata positive potential of about 500 volts and which makes the electronsdiverge somewhat and pass through the aperture 22 therein. A similarelectrode 24 is at a lower positive potential of about 200 volts whichcauses the electrons to be converged somewhat as they are attractedthereto. The electrons pass through the aperture 26 in the electrode 24and are then attracted by the high positive potential on electrode 30which is at about the same potential as the electrode 26. Again theelectrons are caused to diverge somewhat so that as they pass throughthe aperture 28 therein they emerge therefrom as the beam 34 containingsubstantially parallel electron rays. The electrons then pass throughthe first aperture 37 of an electrode 43 which may consist of twocylindrical members having respective apertures 37 and 39 and havingrespecto provide an electube in which electron-optical essentiallyeliminated.

tive flat end portions 41 and 42. Both parts of this structure areconnected electrically to a voltage of about +5000 v. This electrodetends to minimize the variation of the electrostatic field in theretroflexed portion of the tube neck thereby minimizing any deflectionof the beam resulting from the changein voltage that is applied toelectrode 40 whose operation will now be explained.

It will be noted that constant potentials have been applied to all theelectrodes mentioned thus far so that there is no modulation of the beamproduced thereby. Consequently the beam current density as it leaves theaperture 37 will remain constant and the emission over the emissivesurface of the cathode will be maintained constant. i

In accordance with my invention a mirror electrode 40 is located in thepath of the beam of electrons 34. It is connected to a source 38 of acontrol voltage wave which may comprise, for example, a source of asignal modulated at video frequencies .if the cathode ray tube is to beemployed for reproducing television images. The control wave maytypically have a voltage swing of about to volts which is much less thanis requiredin a conventional electron gun. This is due to the fact thatthe modulating electrode does not have .to act as a screen with respectto the field created by the high positive voltages on the subsequentaccelerating electrodes (as the control grid does in aconventionalcathode ray tube) and therefore does not require theapplication thereto of a relatively large negative voltage to cut-off orreduce the current in the beam to-a minimum.

As the potential on the electrode 40-goes more positive, more of theelectrons in the incident beam 34 will be attracted thereto as shown atthe numeral 50 and will be absorbed thereby,.and fewer will be reflectedto form the reflected beam 35. Conversely, when the signal goes morenegative (less positive), more of the electrons in the original beam 34will be reflected to form the beam 35 .and'fewer will be absorbed by theelectrode 40.

The electrons in the reflected beam 35 then pass through the aperture 39of electrode 43 and traverse the neck of the tube untilthey aresubjected to the focussing field of a conventional focussing coil 33 (ifmagnetic focussing is employed), forexample, and then are deflected by aconventional yoke 32 and swept over the screen 36 to which a highpositive voltage is applied.

The'beam-forming electrodes shown in the sole FIG- URE may alternativelybe of other types. In certain applications where raher largebeamcross-sections can .be tolerated, the beam-generating andformingelectrodes may be supplanted by a so-called Pierce gun which isdescribed in lectron Optics, by O..Klemperer (Cambridge UniversityPress, Second Edition, 1953) at page 270, et seq. This type of electrongun has found utility in ,microwave devices where large -beamcross-sections can The tolerated. .If a small beam cross-sectionis-desired, '-as in television-typecathode ray tubes, manufacture of anappropriatePierce gun-for-this purpose would pose a considerable problemfrom the point of view of the tolerances involved.

his a fact known to workers in electron-optics that electron lensesformed by static fields invariably introduce positive sphericalaberration. Inordinary optical applications such aberration may becompensated by a corrective lens which produces a negative sphericalabernation. In electron-optical applications there is no way ofproducing an electron-optical lens/having a negative sphericalaberration characteristic. It has been known,

however, that spherical aberration in electron-optical systemsmay becorrected somewhat by the use of an electron mirror. It is thereforepossible by employing an electron mirror as the modulating electrode tocorrect for spherical aberration that is introduced either by anelectron lens in the cathode region or by a lens located in the regionin which electrons are focussed, i,e.,'in the focus field ofaconventional cathode ray tube. Thus, if the cathode-grid lens produces apositive spherical aberration I the modulating mirror electrode 40andthe field produced thereby can be constructed to compensate thereforby introducing a certain amount of negative aberration.

' Reference is made'to "Electron Optics, supra, at pages 152, 153, andthe Ramberg article (1949) listed on page 455 thereof. It is evenconceivable that the mirror 40 may be so constructed as toover-compensate for the spherical aberration introduced in theelectron-gun region so as simultaneously to correct for sphericalaberration introduced into the reflected beam 35 by subsequent focusfields or other lenses.

With the use of an electron mirror it is also possible to obtaincorrections of the electron-optical analogue of the optical aberrationknown as chromatic aberration." Electrons emitted from the cathodepossess different velocities and therefore react differently to theeiicct of the electron lenses thereupon. In some cases it may beadvisable to shape the electron mirror either physically or by theapplication of appropriate voltages thereto in order to compensate forthis type of aberration. See Electron Optics, supra, page 174.

While the present invention has been shown in a form in which electronsemanating from the beam portion of the electron-gun trace essentiallyparallel electron rays this is not an indispensable requirement. Hence,conventional types of beam-generating and accelerating structures may beemployed, but it is believed that optimal operation of an electron-gunconstructed according to the present invention is more likely to beachieved when the electron rays approaching the electron mirror aresubstantially parallel since the effect of the electron mirror is morelikely to be uniform thereupon than if they are not.

I claim:

1. An electron gun system for an electron discharge device comprising:means including a cathode for producing an electron beam ofsubstantially constant intensity, means for causing said electrons ofsaid constant-intensity beam to travel in substantially parallel pathsextending in a first direction, an electron mirror electrode disposed insaid electron paths, means for applyinga relatively low and varyingpotential positive with respect to the potential of said cathodeuniformly to said electron mirror electrode, thereby to cause saidmirror electrode to intensity-modulate said beam by absorbing some of'said electrons and reflecting others in a second direction as afunction of said varying potential, electrode means near said mirrorelectrode, said electrode means having first and second apertures andbeing positioned to permit electrons approaching said mirror electrodeto pass through said first aperture and electrons reflected from v:saidmirror electrode to pass through said second aperture, and means forapplying to said electrode means a high potential positive with respectto said cathode potential thereby to establish an electrostatic fieldbetween said electrode means and said mirror electrode whichsubstantially prevents deflection of said electrons in response tosaidvarying voltage.

2. A cathode-tray system comprising means including a cathode forproducing an electron beam having a substantially constant intensity, asingle electrode positioned in the path of said constant-intensity beam,means for applying uniformly to said electrode a control potentialpositive with respect to the potential of said cathode and having avalue such as to cause said single electrode to modify the intensity ofsaid beam by absorbing some of its electrons and reflecting others inrespective amounts dependent on said value, and an anode for collectingthose electrons of said beam reflected by said single electrode.

3. A cathode-ray system according to claim 2, wherein said singleelectrode is substantially planar and is positioned in said beam path atan acute angle with respect thereto, thereby to cause the path of saidelectrons reflected by said electrode to differ from said path of saidconstant-intensity beam.

4. Acathode-ray system according to claim 2, wherein said means forapplying said control potential comprise means for applying a controlpotential having variations in a range suchas to cause said singleelectrode to inten- References Cited in the file of this patent UNITEDSTATES PATENTS Knoll May 2, 1939 Orthuber Nov. 26, 1940 6 Morton Feb. 3,1942 Smith Oct. 26, 1948 Sziklai Aug. 7, 1951 Giacoletto July 15, 1952Dichter Jan. 24, 1956 FOREIGN PATENTS France Dec. 1, 1943

1. AN ELECTRON GUN SYSTEM FOR AN ELECTRON DISCHARGE DEVICE COMPRISING:MEANS INCLUDING A CATHODE FOR PRODUCING AN ELECTRON BEAM OFSUBSTANTIALLY CONSTANT INTENSITY, MEANS FOR CAUSING SAID ELECTRONS OFSAID CONSTANT-INTENSITY BEAM TO TRAVEL IN SUBSTANTIALLY PARALLEL PATHSEXTENDING IN A FIRST DIRECTION, AN ELECTRON MIRROR ELECTRODE DISPOSED INSAID ELECTRON PATHS, MEANS FOR APPLYING A RELATIVELY LOW AND VARYINGPOTENTIAL POSITIVE WITH RESPECT TO THE POTENTIAL OF SAID CATHODEUNIFORMLY TO SAID ELECTRON MIRROR ELECTRODE, THEREBY TO CAUSE SAIDMIRROR ELECTRODE TO INTENSITY-MODULATE SAID BEAM BY ABSORBING SOME OFSAID ELECTRONS AND REFLECTING OTHERS IN A SECOND DIRECTION AS A FUNCTIONOF SAID VARYING POTENTIAL, ELECTRODE MEANS NEAR SAID MIRROR ELECTRODE,SAID ELECTRODE MEANS HAVING FIRST AND SECOND APERTURES AND BEINGPOSITIONED TO PERMIT ELECTRONS APPROACHING SAID MIRROR ELECTRODE TO PASSTHROUGH SAID FIRST APERTURE AND ELECTRONS REFLECTED FROM SAID MIRRORELECTRODE TO PASS THROUGH SAID SECOND APERTURE, AND MEANS FOR APPLYINGTO SAID ELECTRODE MEANS A HIGH POTENTIAL POSITIVE WITH RESPECT TO SAIDCATHODE POTENTIAL THEREBY TO ESTABLISH AN ELECTROSTATIC FIELD BETWEENSAID ELECTRODE MEANS AND SAID MIRROR ELECTRODE WHICH SUBSTANTIALLYPREVENTS DEFLECTION OF SAID ELECTRONS IN RESPONSE TO SAID VARYINGVOLTAGE.